The present invention relates generally to magnetoresistive sensors, and more particularly, to magnetoresistive sensors that use the longitudinal fields produced by their current conductors to stabilize the sensors.
The formation of domains in magnetoresistive heads has long been a problem in magnetic recording because of the resulting domain-induced Barkhausen noise. Before the magnetoresistive element is used, it is typically initialized into a single domain state by the application of a strong easy axis field. The production of a multi-domain state from the single domain state, or destabilization, has previously been shown to result from longitudinal fields that oppose the magnetization direction. For example, see articles by C. Tsang et al, in Journal of Applied Physics, Vol. 53, at page 2602 (1982), and N. Smith, in Journal of Applied Physics, Vol. 63, at page 2932 (1988). These longitudinal fields arise from a number of sources. In the model proposed by Tsang et al., the longitudinal fields are produced by the magnetic poles at the ends of the magnetoresistive element. During application of a transverse field, these demagnetizing fields promote a buckled magnetization structure. This structure relaxes to a multi-domain state when the transverse field is reduced. The model by Smith explains the production of multi-domain states by utilizing the longitudinal field that results from a misalignment of the external field. In both studies the presence of a longitudinal field that opposes the magnetization in the single domain state destabilizes the magnetoresistive element.
An additional source of a longitudinal field in conventional magnetoresistive heads arises from the sense current used to measure the resistance of the magnetoresistive element. The influence of these currents on magnetoresistive behavior has previously been discussed in barber pole magnetoresistive heads. For example, see the articles by J. S. Y. Feng, et al. in IEEE Transactions on Magnetics, Vol MAG-13, at page 1466 (1977), and W. Metzdorf et al., in IEEE Transactions on Magnetics, Vol MAG-18, at page 763 (1982). In these barber pole heads, conductors are placed on top of the magnetoresistive element at an angle to the easy axis. A vertical component of the current exists in the conductor and produces a longitudinal field along the easy axis that helps to stabilize a single domain state. However, for narrow track width applications the slanted conductors in barber pole configurations are not desirable and instead the conductors are brought in perpendicular to the magnetoresistive element. However, even this simple configuration is affected by the fields from the currents. These fields are present on either side of the track and they can either stabilize or destabilize the single domain state in the magnetoresistive sensor depending on the conductor configuration and the direction of the magnetization.
One of the primary mechanisms for instability in magnetoresistive sensors is due to the placement of contact leads. These contact leads provide current to the magnetoresistive sensors and they are typically placed on the same side of the magnetoresistive sensor. The currents flowing through the contact leads produce longitudinal fields that are aligned in opposite directions on either side of the active area of the sensor. Unfortunately, these opposing longitudinal fields produce instabilities in the magnetoresistive sensor because their orientations favor having the magnetization directed in opposite directions on either side of the active area. Regions of oppositely directed magnetization typically form domain walls between them. This tendency to form domain walls is further increased when the magnetization is oriented towards the transverse direction, a condition that occurs in magnetoresistive sensors that are biased, or during normal operation when sensing magnetic fields.
Tests were performed on a magnetoresistive sensor that was fabricated with its current leads on the same side of the magnetoresistive element. When operating in a low current condition, which implies a weak magnetic field, the response curve was smooth. However, when operated at a typical standard operating current of 25 mA, the fields associated with the current are sufficient to introduce severe instabilities in the response curve.
The currents in the contact leads of the magnetoresistive head produce fields that influence the behavior of the underlying magnetoresistive elements. In the conventional lead configuration the current enters and leaves the magnetoresistive elements on the same side of the element. In the region where the conductor overlaps the magnetoresistive element, a field is produced that has a component that is along the easy axis of the magnetoresistive element. The direction of this longitudinal field is in opposite directions on either side of the track and this destabilizes the magnetoresistive element. Barkhausen noise results from the multi-domain states that are produced.